RE: a problem in reading TFile

Hello Liang,      

From: owner-roottalk_at_pcroot.cern.ch on behalf of Zhijun Liang Sent: Wed 10/11/2006 9:29 AM
To: roottalk_at_pcroot.cern.ch
Subject: [ROOT] a problem in reading TFile

hi :
I found a problem in reading TFile
I want to read a object from a root file ,but meet some error . here is the code :
{
TFile f("hsimple.root")
TH1F *hist=(TH1F *) f->Get("hpx"))
TH1F *hpx1=hpx->Clone("hpx")
f.Close()
hist->Draw()
}

and the "hsimple.root " is from tutorial directory. error information appears

• Break *** segmentation violation Generating stack trace... 0x02fff4f0 in <unknown> from /lib/libc.so.6 0x0086efb6 in G__ExceptionWrapper + 0x42 from /home/zliang/root/lib/libCint.so 0x008fed5b in G__call_cppfunc + 0x2a7 from /home/zliang/root/lib/libCint.so

do you know what is the problem ?

It is not a problem. It is the ROOT feature.  

The TFile object "owns" its histogram objects including all objects created "somewhere" by your code , Those are not connected "explicitly" with your file are owned also . As soon as you close the file all histogram objects those belong to the file are deleted. As result your "hist" and "hpx1" pointers point to the "dead" objects leading to the program crash.  

To avoid the crash "disconnect" the histogram from the file with SetDirectory method:  

 hpx1->SetDirectory(0);  

should do the trick. In fact you may not call "Clone" at all.  

hist->SetDirectory(0) ;  

should be sufficient.  

Hope this helps.

          Valeri

thank you
best regards
Liang Received on Wed Oct 11 2006 - 16:06:42 MEST